Nitrogen oxides (e.g., NOx) may be processed in a vehicle exhaust system via a selective reduction catalyst (SCR). The SCR reduces the NOx into N2 and H2O. In some systems the SCR may co-operate with a reductant such as ammonia (NH3) to reduce the NOx when there is sufficient temperature within the SCR. The ammonia may be injected to the SCR from a storage tank via an injector. However, if excess ammonia is delivered to the SCR via the injector, ammonia may pass through the SCR and enter the atmosphere. Models have been developed to estimate the amount of ammonia stored within a SCR so that ammonia injection may be stopped before ammonia slips past the SCR. Nevertheless, ammonia may slip past an SCR due to modeling and system input (e.g., feedgas NOx, injector errors, urea quality) errors. In addition, the NH3 slip past the SCR is wasted urea.
The inventors herein have recognized the above-mentioned disadvantages and have developed a system for controlling and reusing the vehicle exhaust system NH3 slippage, comprising: a first SCR positioned in a vehicle exhaust system downstream of an engine; a second SCR positioned in the vehicle exhaust system; and a controller including instructions to direct NH3 slippage from the first SCR to the second SCR, the controller including additional instructions to direct exhaust gases from the second SCR to the first SCR.
By storing NH3 slippage from a first SCR in a second SCR and directing exhaust gases from the second SCR to the first SCR, it may be possible to reduce NH3 slippage from a vehicle exhaust system to atmosphere as well as recover the slipped NH3 for further NOx reduction in the SCR and lower urea cost. In particular, NH3 that has slipped past the first SCR can be stored in the second SCR and then returned to the first SCR via desorption so that NOx may be additionally processed in the first SCR via the slipped NH3. In this way, it may be possible to better utilize NH3 that has been injected to the exhaust system to reduce NOx.
The present description may provide several advantages. Specifically, the approach may improve NH3 utilization in a vehicle exhaust system. Further, the approach may reduce an amount of NH3 emissions from a vehicle. Further still, the approach may also reduce the amount of NH3 used within the vehicle exhaust system since SCR NH3 can be recycled back to a primary SCR 70.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.